A conventional variable cycle gas turbofan engine includes a core engine driving a fan, and a bypass duct surrounding the core engine which is in flow communication with the fan. A conventional bypass valve is disposed at an upstream, inlet end of the bypass duct and is positionable in a closed position which substantially block flow from the fan into the bypass duct under certain conditions in the flight envelope of an aircraft being powered by the engine while allowing flow from the fan to be channeled into the core engine. The bypass valve is also positionable in an open position which allows substantially unobstructed flow from the fan into the bypass duct for bypassing a portion of the fan air around the core engine while allowing the remaining portion of the fan air to be channeled through the core engine during operation of the aircraft at other conditions in the flight envelope.
Conventional bypass valve assemblies are relatively complex and are controlled in accordance with predetermined schedules corresponding to operation in the flight envelope of the aircraft. An exemplary conventional bypass valve assembly includes an annular ring valve which is translatable to open and close an annular inlet to the bypass duct. Conventional linkages and servovalves are used to translate the valve and are operatively connected to the control system of the engine for being responsive to the predetermined schedules contained in the control system for opening and closing the bypass valve at various conditions in the flight envelope.
In the open position, the bypass valve must provide for substantially unobstructed flow into the bypass duct for reducing or minimizing pressure losses therefrom which would decrease performance of the engine and reduce the cooling ability of the bypass air channeled in the bypass duct. The bypass air is typically used to improve cruise SFC and to cool downstream structures in the engine, such as, for example, a conventional augmentor and variable area exhaust nozzle, and any pressure losses due to the bypass duct would have to be accommodated, typically by increasing pressure in the bypass duct which decreases engine performance. Furthermore, the bypass valve must also provide for substantially unobstructed flow and smooth transition into the bypass duct to prevent or minimize any backpressure on the fan which would undesirably reduce stall margin of the fan.
The bypass valve in the form of a mode selector valve is typically positioned between a fully open position and a fully closed position for double or single bypass operation of an exemplary double bypass engine. In alternate embodiments, the bypass valve may additionally be disposed at intermediate positions therebetween, as required by particular aircraft engine applications. In this way, the bypass ratio conventionally represented by the total engine airflow divided by the core engine airflow may be varied during operation of the aircraft engine.
A significant problem associated with the variable geometry required for positioning bypass valves is the availability of mounting space, and correspondingly, the amount of allowable weight for the bypass valve system. Typically, little axial and radial envelope is available in conventional augmented turbofan engines due to the close proximity between the fan, compressor rotors, and external gear box for the bypass valve system. Without available space, the engine must be redesigned for having a larger diameter and longer axial length for accommodating the required bypass valve system. Increased radial and axial size of a gas turbine engine and the corresponding increase in weight, is undesirable since it leads to additional weight and penalty losses for the overall engine.
Furthermore, conventional bypass valve systems typically require rigging, or adjustments at assembly to ensure coordinated movement and full travel of parts. Rigging increases assembly time and costs associated therewith.